The present invention is a secondary air supply system for an internal combustion engine. In a given engine, one or more pistons reciprocate in cylinders, and the exhaust gas resulting from the combustion process is exhausted into an exhaust system. If sufficient oxygen is available in the exhaust gas, then unburned hydrocarbons and carbon monoxide contained in the exhaust gas may be further oxidized in the exhaust system, thereby reducing such undesirable emissions. The exhaust system may contain a catalytic converter to catalyze, or promote, such oxidation reaction, or, in a process known as thermal oxidation, such oxidation reaction may be induced without a catalyst if the exhaust gas is of a sufficiently high temperature. The required excess oxygen for such oxidation reaction is furnished by injecting air into the exhaust system immediately downstream from the exhaust port and upstream from any catalytic converter or muffler. Such air is referred to as secondary air, in contrast to the air taken in by the engine and mixed with fuel, which is referred to as primary air.
Four-cycle engines having a closed crankcase produce crankcase gases during operation. Such crankcase gases principally result from combustion gases escaping past piston rings, and from the vaporization of moisture and fuel which enter the crankcase. Unless such gases are vented, increasing pressure in the crankcase will cause the engine to lose power and the oil seals and gaskets to leak, and will force lubricating oil in the crankcase to leak past piston rings and into the combustion chamber. In addition, failure to vent such gases will result in premature engine oil contamination and consequent accelerated wear on internal engine parts. Thus, four-cycle engines require a crankcase breather system to allow venting of such crankcase gases. All four-cycle engines for which the present invention is intended, particularly inexpensive single-cylinder utility engines, incorporate some form of crankcase breather system.
In a two-cycle engine, a reed valve assembly is usually employed to control the flow of air and fuel mixture into the closed crankcase of the engine. Such reed valve assembly is attached to the intake port of the engine block or case.
Secondary air is generally introduced into the exhaust system of an internal combustion engine in one or more of the following ways:
(i) A siphon arrangement employing a one-way valve, whereby negative pressure pulses in the exhaust gas flow draw in air under ambient atmospheric pressure. In other words, the flowing exhaust gas sucks in the secondary air through a one-way valve fitted to the exhaust system for that purpose. PA1 (ii) A pump mechanically driven by the engine pumps the secondary air into the exhaust system. PA1 (iii) A diaphragm-type pump, operated by pressure pulsations produced by one or more engine sources, pumps the secondary air into the exhaust system. PA1 (i) introduces a sufficient quantity of secondary air into the exhaust system in proportion to engine speed to permit the oxidation of undesirable exhaust emissions; PA1 (ii) does not induce a net power loss on the engine nor increase fuel consumption; PA1 (iii) is simple and inexpensive to produce, and can be easily fitted to a wide variety of utility engines with little or no disassembly of or modification to the engine or to the vehicle or equipment incorporating the engine; PA1 (iv) does not interfere with the venting of crankcase cases through the crankcase breather system currently fitted to a four-cycle engine; and PA1 (v) through biasing of the diaphragm in a diaphragm air pump, provides an improved means of supplying secondary air by compensating for the intensity differential between the positive and negative pressure pulses in the engine crankcase.
The first type of secondary air supply system mentioned above (the one-way siphon valve) is easy and inexpensive to construct, but has a major drawback in that the amount of secondary air introduced decreases as the amount of exhaust gas flow increases. The result is that at operating conditions other than light engine throttle, there will be insufficient secondary air to permit complete oxidation of undesirable engine emissions. Examples of such air siphon methodology are disclosed in U.S. Pat. Nos. 5,338,903 and 5,339,629.
The second type of secondary air supply system mentioned above (the engine-driven pump) is capable of supplying air in proportion to the amount of engine exhaust gas. Such a system, however, has several major drawbacks. It is expensive relative to other available methods of secondary air supply, and it will result in a net power loss of the engine. In addition, such a system would require major design changes to a utility engine and the vehicle or equipment of which such an engine is a part. These are especially critical issues for small single-cylinder utility engines, which tend to be low-powered and inexpensive, and for which design changes to either the engine or the incorporating equipment could be prohibitively expensive.
The diaphragm air pump (the third system mentioned above) solves the problems associated with the siphon arrangement and the engine-driven air pump. One example of such a diaphragm pump, as driven by engine crankcase pressure pulsations, is disclosed in U.S. Pat. No. 5,197,282. One major drawback of the secondary air supply system disclosed in such patent, however, is that it requires a fitting to be installed in the engine case of an engine unit in order to communicate the crankcase pressure pulsations to the diaphragm air pump. Fitting such a secondary air supply system during the manufacturing of a utility engine would necessitate a design change to the engine case or an extra manufacturing step. With respect to an in-service engine, installing such a system would necessitate a machining step on the engine case and disassembly of the engine in order to remove metal shavings and prevent damage to internal engine parts. Such extensive installation procedures would drastically inhibit the widespread adaptation of such a secondary air supply system to in-service utility engines.
Another major drawback of the secondary air supply system disclosed in U.S. Pat. No. 5,197,282 is that it supplies an insufficient quantity of secondary air when used with a four-cycle engine having a crankcase breather system. Such breather systems employ some form of one-way airflow valve for the purpose of venting crankcase gases and maintaining a slight negative pressure in the engine crankcase. When the crankcase experiences a negative pressure pulse due to movement of the engine's piston in the cylinder, the breather system valve closes and the crankcase is negatively pressurized. When the crankcase experiences a positive pressure pulse due to movement of the piston, the breather system valve is opened and the crankcase gases are expelled through the breather system venting means. Thus, the positive crankcase pressure pulse is largely dissipated through the crankcase breather system, providing a weak or inadequate pressure pulse to the diaphragm air pump disclosed in U.S. Pat. No. 5,197,282.
The prior art also reveals secondary air supply diaphragm pumps driven by the combination of negative pressure pulses from the engine induction and the exhaust gas flow (U.S. Pat. No. 3,498,054), and driven by the engine induction pulses alone but augmented by a siphon valve fitted to the exhaust system to supply additional secondary air (U.S. Pat. No. 4,085,586). The major drawbacks of such combination systems are the complexity and expense of fitting such systems to small, inexpensive single-cylinder utility engines, together with the inadequate quantity of secondary air supplied due to the relatively weak intensity of the induction and exhaust pulsations from such engines.
Another crankcase actuated diaphragm air pump system for supplying secondary air is disclosed in U.S. Pat. No. 4,096,692. This complex system is intended for multicylinder engines, and does not address the problem of a significant intensity differential between the positive and negative pressure pulses in the crankcase due to the operation of a crankcase breather system. This system employs a double spring means operating on the diaphragm not for the purpose of compensating for said pressure intensity differential, but for the purpose of having the springs' resonant frequency determine the engine rotational speed range wherein the pump is operational.
As explained hereinafter, the present invention discloses a crankcase actuated secondary air supply system consisting of (i) an actuator plate to communicate crankcase pressure pulsations to an external diaphragm air pump without the need to modify or disassemble the engine case (as disclosed and allowed in U.S. patent application Ser. No. 08/645,554), and (ii) a diaphragm air pump wherein the displacement of the diaphragm is aided by a spring or other biasing means designed to compensate for the intensity differential between the positive and negative pressure pulses in the crankcase. The relatively inexpensive nature of the utility engines for which the present invention is intended renders secondary air injection the most cost-effective means of significantly reducing undesirable exhaust emissions from such engines. The type of electronic management systems currently found on more expensive engines, such as automotive engines, which provide for careful monitoring and precise control of the engine's air-fuel ratio and exhaust emissions, are too complex and costly to be fitted to such inexpensive utility engines.